Reservoir injection systems using very high injection pressures and high switching speeds are being increasingly used in the fuel supply of combustion engines. Such injection systems are known as common rail systems (for diesel engines) and HPDI injection systems (for Otto engines). In these reservoir injection systems, the fuel is fed into a high-pressure reservoir common to all cylinders. The fuel is then injected into the individual combustion chambers of the combustion engine by means of fuel injectors.
The fuel injector generally includes an injection valve that is opened and closed hydraulically by a servo valve in order to precisely set the timing of the injection process in the combustion chamber.
The servo valve is actuated by an electrically triggered actor. The use of piezoelectric actors has proven to be particularly effective in achieving sufficiently short switching times. In this kind of piezoelectric actor, longitudinal extension that is transferred to the servo valve which then again opens or closes the injection valve is brought about by the application of electrical voltage. For the longitudinal extension of the piezoelectric actor, which is in the μm range, to be able to actuate the servo valve, this longitudinal extension is generally either mechanically assisted by lever gears with bearings in fuel or hydraulically amplified by a pressure chamber.
A fuel injector with a piezoactor and hydraulic amplification is described, for example, in U.S. Pat. No. 5,779,149.
In order to be able to attain the high switching speeds required for optimal combustion timing and small injection amounts with the fuel injector, it is necessary to adjust the fuel injector very precisely.
This applies particularly to the idle stroke between the piezoelectric actor and the, servo valve. On the one hand, the idle stroke should be as small as possible to have constantly defined conditions and to keep the dynamic loads low. On the other hand, there must be minimal play between the actor and the adjusting element in order to avoid malfunctions during operation.
The setting of the idle stroke in the fuel injector has previously be done in such a way that the exact configuration of the individual components of the fuel injector and especially spaces between them are determined by computer from the dimensions of these components.
For that purpose, each component has to be measured at considerable expense. After measurement, the idle stroke is then set by adjustment disks placed between the injector housing and the actor or the servo valve; these disks must have only very close tolerances and are therefore very expensive to manufacture.
To check the adjusted idle stroke, it has previously been necessary to assemble the fuel injector completely and to test it under operating conditions. If malfunctions are found, the fuel injector must be completely broken down again into its individual parts after the test run and possibly reworked or the adjustment disks must be replaced.